You're right, they are not all the same. Nozzle opening pressures can vary quite a bit for any number of reasons. I have seen some as low as 1800 psi and others at 3000 plus. Most tractors will fall in the 2200 to 2800 psi range
Indirect injection is around 1500 PSI and common rail injection around 15000 PSI and yes it varies with different systems. at least thats what i have read.
All depends on your pumps and injectors. I have a modified LBZ D-Max and I have a custom set of injectors and when in order to have keep fuel going at full HP I'm running close to 40,000psi. In the last few months since I've updated my fuel system I've doubled my HP and little over tripled my torque